Industrial waste waters that must be treated before their disposal or reuse often include contaminated waters comprising polar and/or apolar organic compounds, and/or heavy metal salts, and/or oil dispersed or in emulsion. Said waters may come from a variety of industries such as, for example, aluminium and steel production industries, chemical and/or petrochemical industries, automotive industries, oil industries.
In particular, oil industries, both during the extraction and during the refining, produce large amounts of water. For example, during the extraction, both the production water extracted along with the oil and the injection water deriving from the return to the surface, along with hydrocarbons, of the water pumped into the well for keeping pressure values to adequate levels, are produced.
Typical contaminant compounds present in waste waters deriving from oil industries, in particular in production waters and in refinery waste waters (e.g., cooling waters, wash waters, refinery ground waters), and in waste waters deriving from petrochemical industries (e.g., cooling waters, wash waters, ground waters from petrochemical industries), are shown in Table 1.
TABLE 1CLASSES OFCONTAMINANTEXAMPLES OF CONTAMINANTCOMPOUNDSCOMPOUNDSPolar and apolar organicAliphatic hydrocarbons; carboxyliccompoundsacids; optionally halogenated phenols;optionally halogenated aromaticcompounds; glycols; alcohols; ethers(MTBE, ETBE); aldehydes; ketones;halogenated solvents.Oil dispersed or in emulsionPolyaromatic hydrocarbons; alkyl-phenols.Dissolved mineralsSalts containing Na+, K+, Ca2+, Mg2+,Ba2+, Sr2+, Fe2+, as cations, and Cl−,SO4−, CO32−, HCO3−, as anions.Heavy metal salts such as Cd, Cr, Cu,Pb, Hg, Ni, Ag, Zn.NORM (natural radioactivesubstances).Chemical additivesCorrosion and scale inhibitors;biocides; emulsifiers; anti-foamagents.Suspended solidsLimescale; waxes; microorganisms;asphaltenes; iron oxides.Dissolved gasesCarbon dioxide; oxygen; hydrogensulphide.
Treatments for the removal of the above-mentioned contaminant compounds are known in the art. Examples of said treatments are shown in Table 2.
TABLE 2TREATMENT TYPESTREATMENT EXAMPLESPhysicalAdsorption on active carbon (GAC),zeolites, resins; dissolved airprecipitation; C-tour; cyclones;evaporation; sand filters;electrodialysis; freezing-defrosting/evaporation; treatment withmembranes (MF, UF, NF, RO).ChemicalPrecipitation; oxidation;electrochemical processes; photo-catalytic processes; Fenton process;ozone; room temperature ionic liquids;emulsifiers.BiologicalAerobic processes; anaerobicprocesses.
The above-mentioned physical and/or chemical treatments are generally carried out in offshore plants where spaces are limited and compact technologies can be used. However said treatments, besides having high costs, may exhibit some drawbacks. In fact, said treatments are not always totally effective in removing both the above-mentioned polar or apolar organic compounds and the above-mentioned dissolved minerals, as well as the above-mentioned oil dispersed or in emulsion.
On the other hand, the above-mentioned biological treatments are generally carried out in onshore plants. However, said biological treatments, generally less expensive and more effective compared to the above-mentioned physical and/or chemical treatments, cannot always be carried out, in particular, in the presence of:                high salt concentrations that strongly inhibit the activity of the micro-organisms used;        substances that are toxic for the biomass (e.g., benzene);        organic substances that are hardly biodegradable (e.g., MTBE).        
Moreover, said biological treatments generally require the management of large volumes of muds produced.
Finally, further problems may result from a secondary pollution due to the use of chemical additives that may be used in order to control the above-mentioned chemical, physical and/or biological treatments.
Treatments of contaminated water using microporous alumino-silicates, i.e. zeolites, are described in the art.
For example, US patent application 2004/0206705 describes a process for the treatment of water contaminated by apolar compounds characterised in that the treatment is performed on contaminated ground water and consists in making the water pass through a permeable reactive barrier (PRB), placed in situ perpendicular to the ground water, wherein the reactive means consists of one or more apolar zeolites having a silica/alumina ratio higher than 50 and having structural channels (i.e. pores) of a size similar to that of the molecules of the contaminant compounds. The above-mentioned process is said to be capable of removing the contaminant apolar compounds effectively and selectively compared to the mineral salts normally dissolved in water.
U.S. Pat. No. 7,341,665 describes a process for the treatment of water contaminated by apolar organic compounds and/or by heavy metals which consists in circulating the water through a system comprising at least two types of zeolites having a silica/alumina ratio higher than 50, places in a succession, wherein the first zeolite wherethrough the water is made to pass is characterised by a high adsorption capability and by structural channels (i.e. pores) of a size ranging from 7 Å to 50 Å, and the second zeolite is characterised by a high capability of molecule removal with molecular diameter comparable to the dimension of the structural channels (i.e. pores) thereof ranging from 5 Å to 7 Å. The above-mentioned process is said to be capable of removing contaminant apolar organic compounds in an effective manner, both if they are present in small amounts and if they are present in large amounts, thanks to the synergic effect of the two zeolites.
Treatments of contaminated water using membranes are also described in the art.
For example, Visvanathan et al., in the article “Volume reduction of produced water generated from natural gas production process using membrane technology”, published in “Water Science and Technology” (2000), Vol. 41, pages 117-123, describe a process for the treatment of produced water generated from the natural gas production process, comprising sending said produced water to a pre-treatment unit comprising an ultrafiltration membrane (UF), or a nanofiltration membrane (NF), obtaining a permeate and a retentate; sending the permeate obtained from the pre-treatment unit to a treatment unit comprising a reverse osmosis (RO) membrane. The above-mentioned pre-treatment is said to be required in order to prevent the fouling of the reverse osmosis (RO) membrane.
Mondal et al. in the article “Produced water treatment by nanofiltration and reverse osmosis membranes”, published in “Journal of Membrane Science” (2008), Vol. 322, pages 162-170, describe the treatment of produced water co-produced during the production of oil or gas, through a nanofiltration (NF) or reverse osmosis (RO) membrane. In particular, the following membranes have been tested:                NF 270: thin film composite membrane based on piperazine and semi-aromatic polyamide [nanofiltration (NF)];        NF 90: thin film composite membrane based on aromatic polyamide [nanofiltration (NF)];        BW 30: thin film composite membrane based on aromatic polyamide [reverse osmosis (RO)].        
The tests showed a fouling of the membranes. The reverse osmosis (RO) membrane BW 30 produced the best quality permeate compared to the nanofiltration (NF) membranes NF 270 and NF 30.
Ahmadun et al., in the review “Review of technologies for oil and gas produced water treatment”, published in “Journal of Hazardous Materials” (2009), Vol. 170, pages 530-551, describe several treatment techniques for produced water deriving from oil and gas industry. Among these there are described, for example, treatment techniques through microfiltration membranes (MF), ultrafiltration membranes (UF), nanofiltration membranes (NF), reverse osmosis (RO) membranes.
U.S. Pat. No. 5,028,336 describes a method for the treatment of water (e.g., production water deriving from the production of oil or gas) having low pH and containing water-soluble dissolved organic electrolytes, which comprises: raising the pH of said water so as to obtain an alkalized water containing water-soluble dissolved organic electrolytes; subjecting said alkalized water containing water-soluble dissolved organic electrolytes to nanofiltration so as to obtain (i) an aqueous retentate containing a higher concentration of water-soluble dissolved organic electrolytes and (ii) an aqueous permeate containing a lower concentration of water-soluble dissolved organic electrolytes; recovering said aqueous retentate containing a higher concentration of water-soluble dissolved organic electrolytes; and recovering said aqueous permeate containing a lower concentration of water-soluble dissolved organic electrolytes. The above-mentioned treatment is said to be capable of effectively removing the water-soluble dissolved organic electrolytes present in said water.
However, the above reported processes may exhibit some drawbacks. In fact, the above-mentioned processes are not always capable of giving the desired results.
On the one hand, the processes using microporous alumino-silicates (e.g., zeolites) do not allow an effective removal of polar organic compounds having a small number of carbon atoms (e.g., a number of carbon atoms lower than or equal to 8), in particular in the case of oxygenated polar organic compounds such as alcohols, glycols, aldehydes, ketones and carboxylic acids. Moreover, the use of said microporous alumino-silicates does not allow an effective removal of heavy metal salts and of the oil dispersed or in emulsion.
On the other hand, the processes using membranes do not always allow an effective removal of apolar organic compounds such as, for example, benzene, ethylbenzene, toluene, xylenes (known as BTEX), which are aggressive towards said membranes. In particular, high concentrations of said compounds (e.g., concentrations higher than or equal to 10 ppm) may cause a depolymerization of the membranes, thus making them unusable for the purpose.